A new species of Acanthopotamon Kemp, 1918 (Decapoda: Brachyura: Potamidae: Potaminae) from northeastern India, with a key to the species of the genus and notes on their distribution in relation to freshwater ecoregions

The potamid genus Acanthopotamon Kemp, 1918 is known from three species, A. fungosum (Alcock, 1909), A. martensi (Wood-Mason, 1875) (type species), and A. panningi (Bott, 1966), and is found in Bangladesh, India, Nepal, and Pakistan. Within this range, the genus is found in the 'Ganges Delta and Plain,' 'Lower and Middle Indus,' and 'Namuda-Tapi' freshwater ecoregions. Here we describe a fourth species, A. horai sp. nov., from northeastern India (Arunachal Pradesh, Assam, and Mizoram) in the 'Middle Brahmaputra' and 'Ganges Delta and Plain' freshwater ecoregions. The new species possesses a unique suite of external and gonopod morphological characters, with the slender and narrowly conical terminal segment of the first male gonopod being particularly diagnostic. A key to the species of Acanthopotamon is provided.